Generation time and seasonal migration explain variation in spatial population synchrony across European bird species

Abstract

Spatial population synchrony is common among populations of the same species and is an important predictor of extinction risk. Despite the potential consequences for metapopulation persistence, we still largely lack understanding of what makes one species more likely to be synchronized than another given the same environmental conditions. Generally, environmental conditions in a shared environment or a species' sensitivity to the environment can explain the extent of synchrony. Populations that are closer together experience more similar fluctuations in their environments than those populations that are further apart and are therefore more synchronized. The relative importance of environmental and demographic stochasticity for population dynamics is strongly linked to species' life‐history traits, such as pace of life, which may impact population synchrony. For populations that migrate, there may be multiple environmental conditions at different locations driving synchrony. However, the importance of life history and migration tactics in determining patterns of spatial population synchrony have rarely been explored empirically. We therefore hypothesize that increasing generation time, a proxy for pace of life, would decrease spatial population synchrony and that migrants would be less synchronized than resident species. We used population abundance data on breeding birds from four countries to investigate patterns of spatial population synchrony in growth rate and abundance. We calculated the mean spatial population synchrony between log‐transformed population growth rates or log‐transformed abundances for each species and country separately. We investigated differences in synchrony across generation times in resident (n = 67), short‐distance migrant (n = 86) and long‐distance migrant (n = 39) bird species. Species with shorter generation times were more synchronized than species with longer generation times. Short‐distance migrants were more synchronized than long‐distance migrants and resident birds. Our results provide novel empirical links between spatial population synchrony and species traits known to be of key importance for population dynamics, generation time and migration tactics. We show how these different mechanisms can be combined to understand species‐specific causes of spatial population synchrony. Understanding these specific drivers of spatial population synchrony is important in the face of increasingly severe threats to biodiversity and could be key for successful future conservation outcomes.

How do life history traits influence the environment’s effect on population synchrony? Insights from European birds and insects

“…We classified each bird species as a resident, short-distance, or longdistance migrant (following Martin et al 2023). Avian species that migrate are usually categorized based on the extent of their movement between breeding and overwintering regions (Rappole, 2013).…”

“…Fast-slow life history: We used generation time as a proxy for classification of bird species along the fast-slow life history continuum (Gaillard et al, 2005, Bjørkvoll et al, 2012, Martin et al, 2023. The fast-slow life history continuum ranges from species with short generation times that are fast-reproducing and short-lived (i.e., fast-lived) to species with long generation times that are slow-reproducing and long-lived (i.e., slow-lived;Stearns, 1983;Gaillard et al, 1989;Galliard et al, 2016).…”

“…Despite the synchronizing effect of environmental autocorrelation on population dynamics, different species present at the same locations and exposed to the same environmental synchrony do not always exhibit the same degree of synchrony in their population cofluctuations (Marquez et al, 2019, Martin et al, 2023. Different responses to the environment and, thereby, the environmental synchrony are often attributed to life history traits, rendering species-specific sensitivity to changes in the environment (Tedesco & Hugueny, 2006, Chevalier et al, 2014, Hansen et al, 2020.…”

“…These taxa are also generally widely distributed, making it possible to study the same species spread across different environments (Jones et al, 2007). Based on Moran's theorem, we predicted that species of birds and insects in environments with higher synchrony would have overall higher spatial population synchrony, but that the effect of synchronized environments would depend on species' life history traits (Martin et al 2023, Marquez et al, 2019. More specifically, we expected that species more sensitive to environmental stochasticity, such as fast-lived species , or specialist species (Dumoulin & Armsworth, 2022), would be more highly synchronized and more influenced by environmental synchrony.…”